1. Field of the Invention
The present invention relates to memory devices based on phase change based memory material, including chalcogenide based materials and other materials, and methods for operating such devices.
2. Description of Related Art
Phase change based memory materials, like chalcogenide based materials and similar materials, can be caused to change phase between an amorphous and a crystalline state by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher electrical resistivity than the generally crystalline state, which can be readily sensed to indicate data. These properties have generated interest in using phase change material to form nonvolatile memory circuits, which can be read and written with random access.
The change from the amorphous to the crystalline, referred to as set herein, is generally a lower current operation in which current heats the phase change material above a transition temperature to cause a transition of an active region from the amorphous to the crystalline phase. The change from crystalline to amorphous, referred to as reset herein, is generally a higher current operation, which includes a short high current density pulse to melt or breakdown the crystalline structure, after which the phase change material cools quickly, quenching the phase change process and allowing at least a portion of the active region to stabilize in the amorphous phase.
Two performance limitations conventionally associated with phase change memory devices are a relatively slow set operation and a relatively long recovery time.
The set operation for conventional phase change memory devices is typically significantly longer than the read and reset operations. See, for example, U.S. Pat. No. 6,545,907. This comparatively slow set operation limits the overall operational speed of the device, restricting the use of phase change based memory circuits as high speed memory.
In addition, it has been reported that the recovery (or relaxation) time required for the threshold voltage and resistance of phase change material to stabilize following a reset pulse having a pulse width of 100 ns is 30 ns or more. See, “Recovery and Drift Dynamics of Resistance and Threshold Voltages in Phase-Change Memories”, by Ielmini et al., IEEE Transactions on Electron Devices, Vol. 54 No. 2, 2 Feb. 2007, pp. 308-315. These relatively long reset pulses and recovery times make phase change memory devices unavailable for use in applications which require a fast read operation following program or erase cycling.
Thus, integrated circuits employing phase change based memory circuits typically also include other types of memory circuits in order to fulfill the memory performance requirements for the various functions of the integrated circuit. These different types of memory circuits are embedded at various locations in the integrated circuit, and typically include SRAM or DRAM memory circuits in order to provide high access speed memory for the integrated circuit. However, integration of different types of memory circuits for the various memory applications in an integrated circuit can be difficult and result in highly complex designs.
It is therefore desirable to provide phase change memory devices and methods for operation which overcome the performance limitations described above and extend their usefulness into applications requiring very high speed operation.